ATLANTA — For the first time, U.S. life expectancy has surpassed 78 years, the government reported today.

The increase is due mainly to falling mortality rates in almost all the leading causes of death, federal health officials said. The average life expectancy for babies born in 2006 was about four months greater than for children born in 2005.

However, America continues to lag behind about 30 other countries in estimated life span, according to World Health Organization data.

Japan is No. 1 on the list, with a life expectancy of 83 for children born in 2006. Switzerland and Australia were also near the top of the list.

“The international comparisons are not that appealing, but we may be in the process of catching up,” a University of Pennsylvania demographer, Samuel Preston, said. He is a co-chair of a National Research Council panel looking at why America’s life expectancy is lower than other nations’.

The new American data, released today, come from the National Center for Health Statistics. It’s a preliminary report of 2006 numbers, based on data from more than 95% of the death certificates collected that year.

Life expectancy is the period a child born in 2006 is expected to live, assuming the mortality trends observed in that year stay constant.

The 2006 increase is due mainly to falling mortality rates for nine of the 15 leading causes of death, including heart disease, cancer, accidents, and diabetes.

“I think the most surprising thing is that we had declines in just about every major cause of death,” Robert Anderson, who oversaw work on the report for the health statistics center, said.

Health statisticians noted declines of more than 6% in stroke and chronic lower respiratory disease (including bronchitis and emphysema), and a drop of more than 5% in heart disease and diabetes deaths. Indeed, the drop in diabetes deaths was steep enough to allow Alzheimer’s disease — which held about steady — to pass diabetes to become the nation’s sixth leading cause of death.

The American infant mortality rate dropped more than 2%, to 6.7 infant deaths per 1,000 births, from 6.9.

Perhaps the most influential factor in the 2006 success story, however, was the flu. Flu and pneumonia deaths dropped by 13% from 2005, reflecting a mild flu season in 2006, Mr. Anderson said. That also meant a diminished threat to people with heart disease and other conditions. Taken together, it’s a primary explanation for the 22,000 fewer deaths in 2006 from 2005, experts said.

American life expectancy has been steadily rising, usually by about two to three months from year to year. This year’s jump of fourth months is “an unusually rapid improvement,” Mr. Preston said.

Life expectancy was up for both men and women, and whites and blacks. Although the gaps are closing, white women continue to have the highest life expectancy (81 years), followed by black women (about 77 years), white men (76) and black men (70). Health statisticians said they don’t have reliable data to calculate Hispanic life expectancy, but they hope to by next year.

Increases in female smoking are a major reason that men’s life expectancy is catching up with the women’s, Preston said. Improvements in the care of heart disease — a major health problem for black Americans — helps explain an improving racial gap, he said.

About 2.4 million Americans died in 2006, according to the report.